1. Technical Field
The present subject matter relates to radio transmission, and more specifically, to compensating for a power amplifier used for radio transmission.
2. Description of Related Art
As digital radio modulation schemes have been developed, such as HD-Radio™ from iBiquity™ and Digital Radio Mondiale™ (DRM), among others, a greater demand has been placed on transmitters, because with even a small amount of distortion in the gain of the power amplifier, multi-carrier inputs will cause high out-of-band spurious signals and harmonics to appear in the spectrum. Orthogonal Frequency Division Modulation (OFDM) also causes the peak RF power can be quite high compared to the nominal average power.
Traditionally, effects of distortion due to gain compression have been offset via the use of piece-wise linear approximations of the inverse of the gain compression applied to the base-band signal as a pre-distortion. The net result of the system appears linear. This was done in the analog realm and diodes were used to generate the power transition points on the gain curve for the pre-distortion. Recently, application of digital pre-distortion applied to the base-band signal has become the norm using fixed tables within the modulator.
To counteract the effects of high peak-to-average power levels, various methods have been utilized making use of the actual phase modulation of the OFDM carriers in such a way to minimize the peaks. Others have made use of various methods of re-modulating the individual symbols contained within the OFDM signal to make the total peak power less than a nominal value. These methods require detailed knowledge of the actual transmitted signal. And in the case of analog frequency modulation (FM) and digital modulation in the same broadcast, requires knowledge of the FM carrier being sent as well.